1. Technical Field of the Invention
The present invention relates to acoustic resonators and, in particular, bulk acoustic resonators. Such resonators are also commonly referred to as Bulk Acoustic Wave, or BAW, resonators.
2. Description of Related Art
BAW resonators are often used to form RF filters in high-frequency front-end circuits, for example in the field of mobile telephony.
For such an application, it is often desirable for the resonators to be fabricated within high-frequency integrated circuits, to exhibit a high selectivity and a relatively high power and to be produced at a low cost.
Resonators are thus often based on the use of surface-wave, or SAW (for Surface Acoustic Wave), devices. Such filters are advantageous in that they exhibit a high selectivity. However, their frequency and their power are relatively limited.
For this reason, the fabrication of resonators using BAW technology has been proposed given that this technology allows the targeted objectives in terms of frequency, power, selectivity and cost of production to be attained. Furthermore, resonators fabricated using BAW technology can be formed directly on a fully integrated circuit, which thus allows fully integrated high-frequency front-end circuits to be fabricated.
However, a BAW bulk acoustic wave resonator is based on the use of a piezoelectric element with two electrodes. The characteristic resonance frequency of the resonator is directly linked to the thickness of the piezoelectric element and of the electrodes covering it. Accordingly, by design, the resonator only comprises a single resonance frequency.
As a consequence, one resonator must be fabricated for each resonance frequency of interest within the device being fabricated.
Thus, for the fabrication of a mobile telephone handset front-end circuit, a specific resonator must be provided for each telecommunications standard used by the handset.
There is a need in the art provide a bulk acoustic wave resonator with an adjustable resonance frequency. There is further a need in the art to provide, at a low cost, resonators of the BAW type capable of being adapted, without modifications, to specifications of various standards of equipment in which they are to be installed.